1. Technical Field
This invention relates to a projector which projects a rectangular image to an external projection plane, such as a screen. More specifically, this invention relates to an image projection method for projecting the image, while correcting a shape of the image projected on the projection plane, and a projector which adopts the method.
2. Description of Related Art
In a field of presentation or video projection, a projector is used that accepts image data from outside and carries out extended image projection to a projection plane, such as an external screen or a wall, based on the accepted image data. Such a projector is provided with a planar image formation panel consisting of a liquid crystal panel or a DMD (Digital Micromirror Device), which forms an image. The projector carries out image projection by projecting to the external projection plane light reflected by the image formation panel, or light which penetrates the image formation panel. Thereby, the image is projected on the projection plane. Hereafter, the image projected on the projection plane is called a projection image, and the image formed by the image formation panel is called a panel image.
Generally, a shape of the projection image that the projector should project is a rectangle. When the projector is arranged so that the light can be perpendicularly projected to the projection plane, the projector can form the panel image to be rectangle shape and projects the light, so that the projector can project the rectangular projection image on the projection plane. However in many cases, the projector cannot be arranged to project light perpendicularly to the projection plane. In such a case, the projector may project light in an oblique direction to the projection plane, from the upper or lower side of the projection plane, or from the right or left side thereof. When the projector projects the light in the oblique direction to the projection plane after forming a rectangular panel image, a traveling distance of light to the projection place differs at both ends of the image, and a magnification of the image differs. Then, the shape of the projection image is distorted from the rectangle. The distortion of this projection image is called keystone distortion. For these reasons, a projector requires a function of the keystone correction which corrects the keystone distortion in order to form the projection image in a rectangle, by projecting light after changing beforehand the shape of a portion of a panel image corresponding to the projection image from the rectangle.
Conventionally, various methods have been proposed for performing the keystone correction with the projector. An exemplary method contains a step that detects the shape of the projection image or distance between the projector and the projection plane, and adjusts the shape of the panel image based on the detection result, thereby correcting the shape of the projection image automatically. An example of this art is described in Japanese Patent Application Laid-Open No. 2003-029714. In the case of using the art of automatic keystone correction case, although the projector needs a sensor to detect data required for the keystone correction, the keystone correction can be performed easily, without making the user to take time and effort. The exemplary method is also used which contains a step that the projector projects an outer frame or points at four corners which show the projection range on the projection plane, and specifies the position of the outer frame or the points of four corners by the user's operation in order to make the shape of the projection range be a rectangle with a predetermined aspect ratio, and by adjusting the shape of a position of the panel image corresponding to the projection range. Thus, the shape of the projection image is corrected. In a case of performing the keystone correction according to the user's operation, although the user needs to take time and effort for the operation, it is possible to use an area of the screen that is the projection plane to the limitation and to adjust the shape of the projection image to a certain extent freely.
By the way, it is known to use various values as the aspect ratio of an image. A horizontal to vertical ratio of 4:3 and 16:9 are used generally. Many conventional projectors comprise the image formation panel whose aspect ratio is set to be 4:3 or 16:9. In either case, the projector can project both of a projection image whose aspect ratio is 4:3 and a projection image whose aspect ratio is 16:9. FIG. 10A and FIG. 10B are schematic views showing how to project a projection image whose aspect ratio is 4:3 by a projector which has an image formation panel whose aspect ratio is 4:3. FIG. 10A shows an input image with an aspect ratio of 4:3 that is input to the projector to project the image whose aspect ratio is 4:3. The projector stores data of the input image, converts the data by keystone correction, deforms the input image, and forms a panel image containing the deformed input image with the image formation panel. FIG. 10B shows the panel image containing the deformed input image. An image region shown in FIG. 10B is a region of the input image deformed through keystone correction. The image region with an aspect ratio of 4:3 corresponds to the projection image projected on a projection plane. Portions of the panel image other than the imaging range are projected with black, for example.
FIG. 11A and FIG. 11B are schematic views showing how to project an image whose aspect ratio is 16:9 with a projector which has an image formation panel whose aspect ratio is 4:3. FIG. 11A shows an input image with an aspect ratio of 16:9 that is input to the projector to project the image.
FIG. 11B shows a panel image. Since the aspect ratio of the image formation panel is 4:3, the projector forms a panel image with the aspect ratio of 4:3, and the formed panel image includes the input image whose aspect ratio is 16:9. In this case, as shown in FIG. 11B, the projector sets offset regions that do not include the input image in the upper and lower portions of the panel image whose aspect ratio is 4:3. Then, the projector generates a displayable region whose aspect ratio is 16:9 in the panel image whose aspect ratio is 4:3, and keeps an input image deformed through keystone correction in the displayable region. An image region shown in FIG. 11B is a region of the input image deformed through the keystone correction. The image region with an aspect ratio of 16:9 corresponds to the projection image projected on a projection plane. For example, portions of the panel image other than the displayable region are projected in black, and the offset regions are not projected.
Thus, the projector with the image formation panel whose aspect ratio is 4:3 can project either a projection image whose aspect ratio is 4:3 or a projection image whose aspect ratio is 16:9 while performing the keystone correction. Similarly, a projector with an image formation panel whose aspect ratio is 16:9 can project a projection image whose aspect ratio is 4:3, by setting offset regions in the right and left portions of a panel image whose aspect ratio is 16:9. Furthermore, when a resolution of an image which should be projected is lower than a resolution of an image formation panel, a projector can project a desired projection image, by setting at the upper, lower, left and right portions of a panel image offset regions which correspond to the pixel that become unnecessary due to the reduction of the resolution.